Elements intentionally added during the smelting process to improve and enhance certain properties of cast steel and obtain certain special properties are called alloying elements. Commonly used alloying elements include chromium, nickel, molybdenum, tungsten, vanadium, titanium, silicon, manganese, aluminum, copper, rare earths, etc. Phosphorus, sulfur, nitrogen, etc. also sometimes play the role of alloys.
(1) Cr
Chromium can increase the hardenability of steel and have a secondary hardening effect. It can improve carbon steel’s hardness and wear resistance without making it brittle. When the content exceeds 12%, the steel has good high-temperature oxidation resistance and oxidation corrosion resistance and also increases the thermal strength of the steel. Chromium is the main alloying element of stainless steel, acid-resistant steel, and heat-resistant steel.
Chromium can improve the strength and hardness of carbon steel in its rolling state, and reduce the elongation and area shrinkage. When the chromium content exceeds 15%, the strength and hardness will decrease, and the elongation and area reduction will increase accordingly. Parts containing chromium steel can easily obtain higher surface processing quality after grinding.
The main role of chromium in the quenched and tempered structure is to improve the hardenability, so that the steel has better comprehensive mechanical properties after quenching and tempering. Chromium-containing carbides can also be formed in carburizing steel, thereby improving the surface resistance of the material. Abrasive. Spring steel containing chromium is not easy to decarburize during heat treatment. Chromium can improve the wear resistance, hardness, and red hardness of tool steel, and has good tempering stability. In electrothermal alloys, chromium can improve the alloy’s oxidation resistance, resistance, and strength.
(2) Ni
Nickel strengthens ferrite and refines pearlite in steel. The overall effect is to increase strength and has no significant impact on plasticity. Generally speaking, for low-carbon steel that does not require quenching and tempering and is used in rolling, normalizing, or annealing states, a certain nickel content can increase the strength of the steel without significantly reducing its toughness. According to statistics, every 1% increase in nickel can increase the strength by approximately 29.4Pa. As the nickel content increases, the yield degree of steel increases faster than the tensile strength, so the ratio of nickel-containing steel can be higher than that of ordinary carbon steel. While nickel improves the strength of steel, it has less damage to the toughness, plasticity, and other process properties of steel than other alloying elements. For medium carbon steel, because nickel lowers the pearlite transformation temperature and makes the pearlite thinner; and because nickel reduces the carbon content at the eutectoid point, compared with carbon steel with the same carbon content, the number of pearlite is larger. The strength of nickel-containing pearlitic ferrite steel is higher than that of carbon steel with the same carbon content. On the contrary, if the strength of the steel is kept the same, the carbon content of the nickel-containing steel can be appropriately reduced, thus improving the toughness and plasticity of the steel. Nickel can improve the resistance of steel to fatigue and reduce the sensitivity of steel to notches. Nickel reduces the low-temperature brittleness transition temperature of steel, which is of great significance for low-temperature steel. Steel containing 3.5% nickel can be used at -100°C, while steel containing 9% can work at -196°C. Nickel does not increase the steel’s resistance to creep and is therefore generally not used as a strengthening element for heat-strengthened steels.
The linear expansion coefficient of iron-nickel alloys with high nickel content changes significantly with the increase or decrease of nickel content. Using this characteristic, precision alloys and bimetallic materials with extremely low or certain linear expansion coefficients can be designed and produced.
In addition, nickel added to steel can not only resist acid, but also resist alkali, and has corrosion resistance to the atmosphere and salt. Nickel is one of the important elements in stainless acid-resistant steel.
(3) Mo
Molybdenum can improve the hardenability and thermal strength of steel, prevent temper brittleness, increase remanence and coercive force, and improve corrosion resistance in certain media.
In quenched and tempered steel, molybdenum can make parts with larger cross-sections deeply and thoroughly quenched, improving the tempering resistance or tempering stability of the steel, so that the parts can be tempered at higher temperatures, thereby more effectively eliminating ( or reducing) residual stress and improve plasticity.
In carburized steel, in addition to the above-mentioned functions, molybdenum can also reduce the tendency of carbides to form a continuous network on the grain boundaries in the carburized layer, reduce the residual austenite in the carburized layer, and relatively increase the surface layer. wear resistance.
In forging die steel, molybdenum can also maintain a relatively stable hardness of the steel and increase resistance to deformation. Resistance to cracking and wear.
In stainless acid-resistant steel, molybdenum can further improve the corrosion resistance to organic acids (such as formic acid, acetic acid, oxalic acid, etc.) as well as hydrogen peroxide, sulfuric acid, sulfurous acid, sulfates, acid dyes, bleaching powder, etc. In particular, the addition of molybdenum prevents the tendency of pitting corrosion caused by the presence of chloride ions.
W12Cr4V4Mo high-speed steel containing about 1% molybdenum has wear resistance, tempering hardness, and red hardness.
(4) W
In addition to forming carbides in steel, tungsten partially dissolves into iron to form a solid solution. Its effect is similar to that of molybdenum. Calculated by mass fraction, the general effect is not as significant as molybdenum. The main uses of tungsten in steel are to increase tempering stability, red hardness, heat strength, and increased wear resistance due to the formation of carbides. Therefore, it is mainly used in tool steel, such as high-speed steel, hot forging die steel, etc.
Tungsten forms refractory carbides in high-quality spring steel. When tempered at higher temperatures, it can alleviate the aggregation process of carbides and maintain high-temperature strength. Tungsten can also reduce the steel’s susceptibility to overheating, increase hardenability, and increase hardness. 65SiMnWA spring steel has a very high hardness after being air-cooled after hot rolling. Spring steel with a 50mm2 cross-section can be quenched in oil and can be used as an important spring that can bear large loads, resist heat (not more than 350°C), and be impacted. 30W4Cr2VA is a high-strength and heat-resistant high-quality spring steel with large hardenability. After quenching at 1050~1100℃ and tempering at 550~650℃, the tensile strength reaches 1470~1666MPa. It is mainly used to manufacture springs used under high temperature (not more than 500°C) conditions.
Since the addition of tungsten can significantly improve the wear resistance and machinability of steel, tungsten is the main element of alloy tool steel.
(5) V
Vanadium has a very strong affinity with carbon, ammonia, and oxygen, forming corresponding stable compounds with them. Vanadium exists mainly in the form of carbides in steel. Its main function is to refine the structure and grains of steel and reduce the strength and toughness of steel. When dissolved into a solid solution at a high temperature, the hardenability is increased; conversely, when it exists in the form of carbide, the hardenability is decreased. Vanadium increases the tempering stability of quenched steel and produces a secondary hardening effect. The vanadium content in steel, except for high-speed tool steel, is generally not greater than 0.5%.
Vanadium can refine the grains in ordinary low-carbon alloy steel, improve the strength, yield ratio, and low-temperature characteristics after normalizing, and improve the welding performance of the steel.
Vanadium is often used in combination with elements such as manganese, chromium, molybdenum, and tungsten in structural steel because it will reduce the hardenability under general heat treatment conditions. Vanadium in quenched and tempered steel mainly improves the strength and yield ratio of the steel, refines the grains, and reduces overheating sensitivity. Because it can refine the grains in carburized steel, the steel can be quenched directly after carburizing without the need for secondary quenching.
Vanadium in spring steel and bearing steel can increase the strength and yield ratio, especially increase the proportional limit and elastic limit, and reduce decarburization sensitivity during heat treatment, thereby improving surface quality. Pentachromium-containing vanadium-bearing steel has high carbonization dispersion and good performance.
Vanadium refines the grains in tool steel, reduces overheat sensitivity, and increases tempering stability and wear resistance, thereby extending the service life of the tool.
(6) Ti
Titanium has a strong affinity with nitrogen, oxygen, and carbon, and its affinity with sulfur is stronger than that of iron. Therefore, it is a good deoxygenating degassing agent and an effective element in fixing nitrogen and carbon. Although titanium is a strong carbide-forming element, it does not combine with other elements to form composite compounds. Titanium carbide has a strong binding force, and stability, and is not easy to decompose. In steel, it can only be slowly dissolved into a solid solution when heated to above 1000°C. Before being dissolved, titanium carbide particles can prevent grain growth. Since the affinity between titanium and carbon is much greater than the affinity between chromium and carbon, titanium is commonly used in stainless steel to fix the carbon in it to eliminate the depletion of chromium at the grain boundaries, thereby eliminating or reducing intergranular corrosion of steel.
Titanium is also one of the strong ferrite-forming elements, which strongly increases the A1 and A3 temperatures of steel. Titanium can improve plasticity and toughness in ordinary low-alloy steel. Because titanium fixes nitrogen and sulfur and forms titanium carbide, it increases the strength of the steel. After normalizing, the grains are refined and carbides are precipitated to form, which can significantly improve the plasticity and impact toughness of the steel. Titanium-containing alloy structural steel has good mechanical properties and process properties, but its main disadvantage is slightly poor hardenability.
Titanium with about 5 times the carbon content is usually added to high-chromium stainless steel, which can not only improve the corrosion resistance (mainly resistance to intergranular corrosion) and toughness of the steel; it can also prevent and improve the grain growth tendency of the steel at high temperatures. Welding properties of steel.
(7) Si
Silicon can be dissolved in ferrite and austenite to improve the hardness and strength of steel. Its effect is second only to phosphorus and stronger than elements such as manganese, nickel, chromium, tungsten, molybdenum, and vanadium. However, when the silicon content exceeds 3%, the plasticity and toughness of steel will be significantly reduced. Silicon can improve the elastic limit, yield strength, and yield ratio (σs/σb) of steel, as well as fatigue strength and fatigue ratio (σ-1/σb), etc. This is the reason why silicon or silicon-manganese steel can be used as spring steel.
Silicon can reduce the density, thermal conductivity, and electrical conductivity of steel. It can promote the coarsening of ferrite grains and reduce the coercive force. It tends to reduce the anisotropy of the crystal, making magnetization easier and reducing magnetic resistance. It can be used to produce electrical steel, so the magnetic hysteresis loss of silicon steel sheets is low. Silicon can increase the magnetic permeability of ferrite, making the steel sheet have higher magnetic induction intensity under weaker magnetic fields. However, silicon reduces the magnetic induction intensity of steel under strong magnetic fields. Silicon has strong deoxidizing power, thereby reducing the magnetic aging effect of iron.
When steel containing silicon is heated in an oxidizing atmosphere, a layer of SiO2 film will be formed on the surface, thereby improving the oxidation resistance of the steel at high temperatures. Silicon can promote the growth of columnar crystals in cast steel and reduce plasticity. If silicon steel cools quickly when heated, due to low thermal conductivity, the temperature difference between the inside and outside of the steel will be large, causing it to break.
Silicon can reduce the welding properties of steel. Because silicon has a stronger ability to combine with oxygen than iron, it is easy to generate low-melting-point silicates during welding, which increases the fluidity of slag and molten metal, causing splashing and affecting welding quality. Silicon is a good deoxidizer. When using aluminum for deoxidation, adding a certain amount of silicon as appropriate can significantly improve the deoxidation rate. There is a certain amount of silicon remaining in steel, which is brought in as a raw material during iron and steel making. In boiling steel, silicon is limited to <0.07%, and when added intentionally, ferrosilicon alloys are added during steelmaking.
(8) Mn
Manganese is a good deoxidizer and desulfurizer. Steel generally contains a certain amount of manganese, which can eliminate or weaken the hot brittleness of steel caused by sulfur, thereby improving the hot workability of steel.
The solid solution formed by manganese and iron improves the hardness and strength of ferrite and austenite in steel; it is also an element that forms carbides and enters cementite to replace some iron atoms. Manganese in steel lowers the critical transformation temperature. It plays the role of refining pearlite and indirectly improves the strength of pearlitic steel. Manganese’s ability to stabilize austenite structure is second only to nickel, and it also strongly increases the hardenability of steel. A variety of alloy steels have been made with manganese content not exceeding 2% and other elements.
Manganese has the characteristics of abundant resources and diverse efficiencies and has been widely used, such as carbon structural steel and spring steel with high manganese content.
In high-carbon and high-manganese wear-resistant steel, the manganese content can reach 10% to 14%. It has good toughness after solid solution treatment. When deformed by impact, the surface layer will be strengthened due to deformation and have high durability. Abrasive.
Manganese and sulfur form MnS with a higher melting point, which can prevent hot embrittlement caused by FeS. Manganese tends to increase grain coarsening and temper the brittleness susceptibility of steel. If the cooling is improper after smelting, pouring, and forging, it is easy to cause white spots in the steel.
(9) Al
Aluminum is mainly used for deoxidation and grain refinement. Promotes the formation of a hard and corrosion-resistant nitrided layer in nitrided steel. Aluminum can inhibit the aging of low-carbon steel and improve the toughness of steel at low temperatures. When the content is high, it can improve the oxidation resistance of steel and the corrosion resistance in oxidizing acids and H2S gas and can improve the electrical and magnetic properties of steel. Aluminum has a great solid solution strengthening effect in steel, improving the wear resistance, fatigue strength, and core mechanical properties of carburized steel.
In refractory alloys, aluminum, and nickel form compounds to improve the smelting strength. Iron-chromium-aluminum alloys containing aluminum have characteristics close to constant resistance and excellent oxidation resistance at high temperatures and are suitable for electro-smelting alloy materials and chromium-aluminum alloys. Resistance wire.
When certain steels are deoxidized, if too much aluminum is used, the steel will produce abnormal structures and tend to promote graphitization of the steel. In ferritic and pearlitic steels, when the aluminum content is high, it will reduce its high-temperature strength and toughness, and bring some difficulties to smelting and pouring.
(10) Cu
The prominent role of copper in steel is to improve the atmospheric corrosion resistance of ordinary low-alloy steel. Especially when used in combination with phosphorus, adding copper can also improve the strength and yield ratio of the steel without adversely affecting the welding performance. Influence. Rail steel (U-Cu) containing 0.20% to 0.50% copper, in addition to wear resistance, its corrosion resistance life is 2 to 5 times that of ordinary carbon rails.
When the copper content exceeds 0.75%, aging strengthening can occur after solid solution treatment and aging. At low levels, its effect is similar to that of nickel but weaker. When the content is high, it is unfavorable to hot deformation processing and causes copper brittleness during hot deformation processing. 2% to 3% copper in austenitic stainless steel can provide corrosion resistance to sulfuric acid, phosphoric acid, and hydrochloric acid and stability to stress corrosion.
(11) RE
Generally speaking, rare earth elements refer to the 15 lanthanide elements (15 elements) with atomic numbers from 57 to 71 in the periodic table of elements, plus scandium number 21 and yttrium number 39, a total of 17 elements. Their properties are close and difficult to separate. The unseparated ones are called mixed rare earths, which are relatively cheap. Rare earth elements can improve the plasticity and impact toughness of forged and rolled steel, especially in cast steel. It can improve the creep resistance of heat-resistant steel electrothermal alloys and high-temperature alloys. Rare earth elements can also improve steel’s resistance to oxidation and corrosion. The antioxidant effect exceeds that of silicon, aluminum, titanium, and other elements. It can improve the fluidity of steel, reduce non-metallic inclusions, and make the steel structure dense and pure.
Adding appropriate rare earth elements to ordinary low-alloy steel has good deoxidation and desulfurization effects, improves impact toughness (especially low-temperature toughness), and improves anisotropic properties. Rare earth elements in iron-chromium-aluminum alloys increase the alloy’s antioxidant capacity, maintain the fine grains of steel at high temperatures, and improve high-temperature strength, thus significantly improving the service life of electrothermal alloys.
(12) N
Nitrogen can be partially used in iron to have the effect of solid solution strengthening and improving hardenability, but it is not significant. Since nitride precipitates on the grain boundaries, it can improve the high-temperature strength of the grain boundaries and increase the creep strength of steel. Combined with other elements in steel, it has a precipitation-hardening effect. The corrosion resistance of steel is not significant, but after nitriding the surface of the steel, it not only increases its hardness and wear resistance but also significantly improves the corrosion resistance. Residual nitrogen in mild steel can cause age brittleness.
(13) S
Increasing the content of sulfur and manganese can improve the cutting performance of steel. In free-cutting steel, sulfur is added as a beneficial element. Sulfur segregates seriously in steel. It is a harmful element that deteriorates the quality of steel and reduces the plasticity of steel at high temperatures. It exists in the form of FeS with a lower melting point. The melting point of FeS alone is only 1190°C, while the eutectic temperature of the eutectic formed with iron in steel is even lower, only 988°C. When the steel solidifies, iron sulfide precipitates at the primary grain boundaries. When steel is rolled between 1100 and 1200°C, FeS on the grain boundaries will melt, greatly weakening the bonding force between grains and causing hot brittleness of the steel. Therefore, sulfur should be strictly controlled. Generally controlled at 0.020% ~ 0.050%. To prevent brittleness caused by sulfur, enough manganese should be added to form MnS with a higher melting point. If the flow rate in the steel is high, pores and looseness will be formed in the weld metal due to the generation of SO2 during welding.
(14) P
Phosphorus has strong solid solution strengthening and cold work hardening effects in steel. Adding it as an alloying element to low-alloy structural steel can improve its strength and atmospheric corrosion resistance of the steel, but reduce its cold stamping performance. The combined use of phosphorus, sulfur, and manganese can increase the cutting performance of steel and increase the surface quality of workpieces. It is used in free-cutting steel, so free-cutting steel also contains relatively high phosphorus. Phosphorus is used in ferrite. Although it can improve the strength and hardness of steel, the biggest disadvantage is that it causes severe segregation, increases temper brittleness, significantly increases the plasticity and toughness of steel, and makes the steel prone to brittleness during cold working, which is the so-called “cold processing”. “brittle” phenomenon. Phosphorus also hurts weldability. Phosphorus is a harmful element and should be strictly controlled, with the general content not exceeding 0.03% to 0.04%.
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